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Back in March 2010, The Mars Exploratory Rover took some images of a strange 'blueberry' like rock formation on 'Chocolate Hills' within the 'Concepcion' crater. Those NASA geeks get to make up names for lots of things.

At the moment, folks don't really know how the blueberries formed.

The composition Opportunity found for the dark coating material fits at least two hypotheses being evaluated, and possibly others. One is that the material resulted from partial melting of blueberry-containing sandstone from the energy of the impact. Another is that it formed from filling of fractures in this type of rock before the impact occurred.

"It's possible that when you melt this rock, the sandstone melts before the blueberries do, leaving intact blueberries as part of a melt layer," Squyres said. "As an alternative, we know that this type of rock has fractures and that the sandstone can dissolve. Long ago, water flowing through fractures could have dissolved the sandstone and liberated blueberries that fell down into the fracture and packed together. In this hypothesis, the impact that excavated the crater did not play a role in forming this material, but split rocks along fractures so the material is exposed on the exterior like a coating."

--- NASA (http://www.nasa.gov/mission_pages/mer/news/mer20100324.html)